The Puccinia psidii fungus causes a fungal disease that typically attacks guava plants, hence the commonly known disease name of "guava rust." But the USDA Agriculture Research Service warns that the fungus doesn't just target guavas, and can cause widespread foliar damage and defoliation in a wide variety of ornamental and fruit shrubs. Prevent your plants from contracting this damaging diseases to save yourself the trouble, time and money of having to treat the fungus once it's established.
Inspect all new plants before transplanting them into your landscape. New plants are a common way of introducing guava rust and other diseases to your landscape. Destroy the plants or return them to the nursery or garden store if you notice symptoms like yellow spore growths on the underside of leaves or a reddish coating on plant stems.
Modify how you water your guava and all other plants. The University of Florida recommends minimizing moisture applied to foliage, as wet leaves attract all types of fungal growth, not just guava rust. Instead, water the plants at their base. If you must water plants using an overhead system, apply irrigation in the early morning so the plants have all day to let the sun burn off the excess moisture.
Prune off leaves and branches that exhibit the symptoms of guava rust as soon as you notice such symptoms. Discard the diseased growths in a sealed plastic bag. Do not leave the vegetation in your landscape, nor should you place it in your compost bin. This helps keep the disease from spreading to uninfected plants.
Spray your plants with a fungicide formulated with sulfur or horticultural oils. This helps kill off any fungal spores, according to the University of Florida. Some gardeners choose to spray non-infected plants as a preventative measure during any weather period where extended humidity lingers for several weeks.